The general use of organic solvents as physical absorbents for acid gases is well known in the art, and among the most successful of such solvents are the ethers of oligomers of polyethylene glycol. U.S. Pat. Nos. 3,737,392 of Jun. 5, 1973 to Ameen et al. and 4,581,154 of Apr. 8, 1986 to Kutsher and Valentine describe the use of dimethyl ethers of oligomers with from 2 to 8 ethylene glycol moieties. W. Woelfer, "Helpful Hints for Physical Solvent Absorption", Hydrocarbon Processing (November 1982) 193-197, shows the use of methyl iso-propyl ethers of the same oligomers. Japanese Patent Laid-Open No. SHO 49(1974)-98,383 discloses the use of ethers of oligomers with 2-10 ethylene glycol moieties, in which one of the etherifying groups is tertiary butyl and the other may range from methyl to butyl.
U.S. Pat. No. 2,139,375 of Dec. 6, 1938 to Millar et al. disclosed the general use of ethers, esters, and mixed ether-esters of polyhydric alcohols and oligomers of polyhydric alcohols in the removal of sulphur containing acid gases from gas mixtures. A specific reference to the use of the "dipropyl" ethers of diethylene glycol (page 3, left column, line 60) and dipropylene glycol (page 3, left column, line 62) occurs in this patent as part of a long list of suitable materials (page 3, line 9 of the left column to page 4, line 30). This reference, however, does not teach any advantage of "dipropyl" ethers over any of the other many possible solvents listed. In fact, the reference teaches that "of the polyhydric alcohol esters, esters, and mixed ether-esters having the same oxygen groups in their molecules, the ones having in their molecules the smallest number of directly linked atoms devoid of oxygen atoms linked thereto possess the greatest absorptive capacity for acid gases, which capacity progressively decreases as the number of directly linked carbon atoms devoid of oxygen atoms increases." (Page 2, right column, lines 66-75) Thus, according to this teaching, propyl ethers should have less capacity than methyl and ethyl ethers.
U.S. Pat. No. 3,877,893 of Apr. 15, 1975 to Sweny teaches the removal of contaminants, including carbon dioxide, from gas mixtures by a process as described generally herein, but with use of "a dialkyl ether of a polyethylene glycol solvent having 1-8 carbon atoms in each alkyl group and 3-8 ethylene units" (column 10, lines 66-68). Dimethyl ethers of polyethylene glycol are described as the preferred solvents (column 4 lines 32-34), and there is no teaching of an advantage for diisopropyl ethers.
U.S. Pat. No. 4,044,100 of Aug. 23, 1977 to McElroy teaches the use of mixtures of diisopropanolamine and dialkyl ethers of a polyethylene glycol. Again, dimethyl ethers are taught as preferred (column 3 lines 15-16).
An object of the present invention is to provide a superior separation process for the separation of carbon dioxide by use of a previously unused solvent which has higher solution capacity for carbon dioxide than the solvents noted above, combined with adequately low viscosity to permit practical operation at temperatures below the freezing point of water, resistance to deleterious reactions with water often present in practical gas mixtures, and sufficiently low gas pressure to prevent uneconomical processing losses of solvent.
The solvent is preferably used in otherwise conventional separation processes which comprise the steps of (a) contacting a gaseous mixture, containing carbon dioxide and at least one other gas, with the solvent at an absorption pressure; (b) separating the resulting gas phase that is relatively depleted in the acid gas from the enriched solvent containing dissolved carbon dioxide; (c) reducing the gas pressure over the enriched solvent to a desorption pressure lower than the absorption pressure, whereby carbon dioxide passes from the solvent phase into the gas phase; (d) separating the desorbed carbon dioxide from the solvent; and (e) recycling desorbed solvent to extract more carbon dioxide from a new quantity of gas mixture. Alternatively, but generally less preferably because of the higher energy cost of heating and cooling large volumes of solvent, the carbon dioxide could be separated by absorption at a low temperature and expulsion from the solvent at a higher temperature.
An example of a specific process to which the present invention is especially well suited is the removal of carbon dioxide from synthesis gas. In a typical process of this type, synthesis gas is contacted with cold solvent in a CO.sub.2 absorber operating at about 27 bars pressure. CO.sub.2 is absorbed from the synthesis gas by the solvent flowing down the absorber. Solvent enriched in CO.sub.2 is taken from the bottom of the absorber and injected into a flash drum at a lower pressure such as 5-10 bars. This results in elimination of most of the hydrogen, methane, and inert gases from the rich solvent into a gas mixture above the solvent. The flashed gas is removed and subsequently may be compressed and recyclced to another absorber or routed to other uses. The solvent, which after this first flashing still contains most of its originally absorbed content of carbon dioxide, is then flashed to approximately 1 bar pressure in a second flash drum. As a result of this second flashing, about 50-70% of feed CO.sub.2 is eliminated from the solvent to gas phase space in the flash drum, from which is it removed and transferred to a CO.sub.2 product line.
If the amount of carbon dioxide thereby recovered is adequate for downstream uses for this gas, the solvent may be regenerated in a stripping column. Air is generally used as the stripping medium in a packed tower with countercurrent flow. Spent air, containing some CO.sub.2, is then vented to the atmosphere from the top of the column. Stripped solvent, containing little or no CO.sub.2, is then recycled to the absorber. The amount of CO.sub.2 which is vented with the spent air is lost in this version of the process.
If a higher fraction of the CO.sub.2 needs to be recovered for use, solvent after the second flash at about 1 bar may be routed to a vacuum flash drum generally operated at 0.3-0.8 bars, before the solvent is routed to an air stripper. The addition of this third flashing operation can increase the CO.sub.2 recovery to as much as 90% of the amount in the feed, if the pressure in the vacuum flash is low enough.
Several modifications of the process described above are within conventional practice. The air stripping column may be replaced by another flash drum into which air is injected cocurrently with the solvent stream. The stripping may be performed under vacuum, thereby substantially reducing the amount of air needed and consequent dilution of the recovered CO.sub.2 with air. Stripping may be accomplished with treated synthesis gas instead of air, with the gas stream exiting from the stripper being recycled to the bottom of the absorber. This scheme can result in nearly 100% recovery of the CO.sub.2 in the feed.
Physical solvents are known to be energy efficient compared with chemical solvents, and in many instances they have proved to be very attractive economically. In general, however, physical solvents used in the past have had a higher viscosity than chemical solvents. If the viscosity is too high, larger absorber, flash, and stripper vessels are needed. This increases capital requirements and average downtime in practical separation plants.